1. Field of the Invention
The present invention relates to a fuel injection control system for an automobile engine and more specifically to a fuel injector driver circuit for controlling a current passing through a fuel injector of an engine.
2. Description of the Prior Art
Generally, in a fuel injection system wherein fuel is fed to an engine by way of injecting pressurized fuel through a fuel injector composed of a solenoid valve, the current passing through the fuel injector is controlled through a driver circuit in order to expand the dynamic range of the fuel injector by giving a high speed operation thereto. In the driver circuits disclosed in the prior art, there are proposed so many techniques in which when an injection pulse indicating an injection timing is inputted, initially a relatively high current is fed to the fuel injector in order to open the fuel injector at high speed and after that it is once deenergized to step down the current value, then the current passing through the fuel injector is controlled so as to keep a low holding current enough to hold the fuel injector in an open state by performing a closed loop control by means of detecting the current passing through the fuel injector. In this type of driver circuit, there is provided a socalled flywheel circuit for discharging counter electromotive force generated in a coil of the fuel injector when the current is shut down. When this flywheel circuit is energized upon switching the current from the valve opening current to the holding current, the falling speed of the current passing through fuel injector is slowed down and additionally the current is decreased along a curved line determined by a a time constant of the coil of the fuel injector. The period needed for switching from the valve opening current to the holding current is called "dead zone" that may cause fuel metering errors and other failures.
Because of this, for example Japanese Unexamined Patent Publication No. Toku-Kai-Sho 60-26136 discloses a technique in which the flywheel circuit is deenergized during the falling period of the current passing through the coil from the high current for valve opening to the holding current. Further, in Japanese Unexamined Patent Publication No. Toku-Kai-Sho 63-55345, the relationship between the flywheel control and the holding current control is disclosed.
In the prior art of the above Toku-Kai-Sho 63-55345, two pairs of comparators are provided in order to compare the voltage of a resistor for detecting the current flowing through the fuel injector with the reference voltage value. In this prior art, after the fuel injection pulse is inputted and the transistor for driving the fuel injector is energized, when the voltage of the resistor for detecting the current through the fuel injector exceeds a first reference voltage set higher than the voltage corresponding to the valve opening current, the above transistor is deenergized to reduce the current through the fuel injector by a signal outputted from one comparator. Further, when the voltage of the above resistor goes down to a level below a second reference voltage value, the current through the fuel injector is controlled so as to come into the predetermined range of the holding current and at the same time the flywheel circuit is switched from the OFF to ON condition by an output signal of the other comparator.
However, in a case where the current passing through the fuel injector is stepped down from the high current for the valve opening while the flywheel circuit is deenergized, the falling speed of that current is so large that it becomes unable to ignore the operational delay of the circuits forming the closed-loop, such as a current detecting circuit, a control circuit for drive current and/or the like. Accordingly, as in the prior art Toku-Kai-Sho 63-55345, when the timing for energizing the flywheel circuit is determined on the assumption that this delay is null or small enough to be able to be ignored, as shown by broken lines in FIG. 4 or illustrated in FIG. 12, the current passing through the fuel injector goes down to a fairly low level for the period from the instance that the current of the fuel injector, namely the drive current thereof is detected by the current detecting resistor to the instance that the drive transistor is energized.
As a result of this, the current passing through the fuel injector becomes lower than the holding current value and finally the fuel injection might be stopped before the fuel injection pulse ceases. Especially in fuel injectors for the direct injection type engine wherein high pressure fuel is injected a large electromagnetic force is needed compared to ordinarily used injectors for the port injection type engine and accordingly the current for valve opening is also high (for example, approximately 10 A). In this case, therefore, when the current is stepped down from a high level while the flywheel circuit is deenergized, the falling speed of current becomes faster and the operational delay of circuits becomes larger than in case of conventional engines. Accordingly, in a case where this prior art is applied to a direct injection type engine, there is a very high possibility that such troubles as injection stall or the like occur.